The present invention relates to a method of and an arrangement for transporting flowable media of changing consistencies in general, and more particularly to the control of the operation of submersible motor pumps employed in such arrangements for the direct transportation of solid substances entrained in a carrier liquid, particularly for use in ocean bottom mining of manganese nodules and the like.
A great variety of mechanical and hydraulic conveying or transporting methods is being resorted to at the present time in ocean bottom mining operations for the conveyance or transportation of the obtained minerals. When it is decided to use hydraulic conveyance or transportation, a choice can still be made between the indirect transportation approach, in which the suspension to be transported and/or the carrier liquid is transported to a higher level by means of locks or tubular chambers, and the direct transportation approach for the mined raw materials. In the direct transportation approach, the solid substances mixed with water flow through a suitably designed centrifugal pump, which is usually constructed as a submersible motor pump. When resorting to this kind of transportation, it was heretofore impossible to avoid clogging of the high-pressure port of the pump with the solid substances under certain circumstances.
The operating point of the transportation system is obtained as an intersection of two characteristic curves by entering the corresponding coordinates into a pressure and flow volume diagram. These curves are the respective characteristic curve of resistance of the riser conduit, the pressure loss of which has a minimum at its operating limit, on the one hand, and the characteristic curve of the centrifugal pump, on the other hand. In dependence on the concentration of the solid substance present at any given instant of time in the riser conduit system, both of these characteristic curves change their shapes. As the solid substance concentration in the carrier liquid present in the riser conduit increases, the pressure loss of this riser conduit goes up. In contradistinction thereto, the pressure differential of the centrifugal pump can either rise or fall as the solid substance concentration in the carrier liquid increases, depending on whether the pressure-increasing influence of the density increase caused by the higher solid substance concentration, or the pressure-reducing influence of the elevation head resulting from the higher concentration, especially at larger grain diameters and smaller specific rotational speeds of the pump, prevails. Depending on the circumstances, the characteristic line or curve of the centrifugal pump can change toward lower pressures with either increasing or decreasing concentration of the solid substance in the carrier liquid, and, consequently, the intersection with the riser conduit characteristic curve can be shifted in the direction toward the operating limit and smaller flow volume. In extreme cases, this can result in a back flow of water and of the solid substance entrained therein through the riser conduit. When this happens, the water can flow through the centrifugal pump in the backward direction, yet the entrained solid substance cannot do so due to the centrifugal action of the impeller of the centrifugal pump. This, of course, eventually results in clogging of the high-pressure port of the pump and of the riser conduit. This, of course, is very disadvantageous, especially in view of the fact that the high-pressure port and the riser conduit can be unclogged only by a very cumbersome and time-consuming operation.